1. Field of the Invention
Applicants' invention relates to the general field of downhole air hammers, also known as percussion drills, but also can be used in the fluid drilling sector when using the hammer assembly. More particularly, it relates to a bit retention apparatus designed to retain and retrieve a broken bit when breakage or “shankage” of the bit shaft occurs. The apparatus retains the broken bit and significantly decreases the chance of losing the broken bit while pulling the drill string out of the well bore.
2. Background Information
When using the percussion drilling assembly, bits can and may be broken due to the high energy levels and rotational torque loads they experience when in use. Along with these two stresses, metal fatigue can be a factor due to the constant impact of the percussion drilling tool. These catastrophic failures often lead to “fishing” jobs which use expensive techniques to retrieve the broken bit from the well bore. Often in these cases, the well bore must be cemented and additional costly measures used to go around the bit left in the hole.
The drill bit is of the type that has larger head section (including a cutting face) at one end; a smaller shank/shaft section on the other end that is used to mate with the Driver sub which delivers rotational energy from the drill string to the bit via splines; and a retaining means for holding the drill bit head to the percussive hammer between the two ends. The standard hammer bit features an external threaded section located just above the head section on the hammer bit. This thread is used when breakage of the bit occurs but still results in a fishing operation to retrieve the bit head. In order to retrieve this bit, one must attempt to “screw” a matching tool over the threaded area for means of retrieving the bit. This method is not only unpredictable, but still results in added expense to the operation.
Once the broken bit is “screwed on,” drawing the broken bit from the well bore can still be a problem. Irregular geographical formations and soil/rock texture can cause the bit to twist and allow the possibility of the bit “unscrewing” while being pulled out of the hole. Thus, similar results can occur even once the threaded bit is attached. This can also happen with current retention systems that incorporate a bit that is pre-screwed on to the shank.
Not only do these above mentioned possibilities exist, other problems can also prevent the broken bit from being pulled out of the hole. For example, due to the abrasive characteristics of the geological formations being drilled through, current retention devices can experience washouts to the retention systems. If abrasion causes the retention device to fail, then the chances of retrieving the broken portion of the bit head reduces over time.
These patented ideas have been utilized in the case of retrieving the broken bit but each has flaws in the design.
In the U.S. Pat. No. 5,065,827 (Meyers, et. Al.), the threaded sleeve has been used for such drilling operations but due to the design concept, the broken bit has been “unscrewed” or “pulled” out of the tool while pulling out of the well bore. The percussion drilling tool is mainly used in deviation control and the possibility of the well bore being “cork screwed” can “unscrew” the broken bit from this system due to the string rotating even while chaining out of the hole. The term chaining out of the hole describes the tripping system where the drill string is being pulled with the table locked to prevent the turning, of the drill string. All rotation of the drill string takes place on the pipe being set back so as to remove it from the well bore. The remaining drill string is thus in suspension unable to move due to the power of the rig.
U.S. Pat. No. 4,726,429 (Kennedy; Feb. 23, 1988) describes a retention device which utilizes an extended driver sub and ball bearing mechanism to retain the bit head to the hammer.
U.S. Pat. No. 5,065,827 (Meyers et al; Smith international; 1991) utilizes a “Screw-On” version bit retention system which shows that the bit head could be retained by a external sleeve that was trapped between the Driver Sub (lower sub that has splines to mate with the bit shank for transmitting the torque of the drill-string to rotate the bit) and the hammer housing on its upper end and was threaded to/below the bit head's retaining thread. This first design has the limitation in that it is possible for the bit head to unscrew from the retainer during the trip out of the hole and thus allow the bit head to fall back to the bottom of the hole.
U.S. Pat. Nos. 5,647,447 and 5,699,867 (Jones; Ingersol-Rand; 1997) utilize a similar retaining sleeve (wear collar) that was trapped between the driver sub and hammer housing on the upper end and was attached to the bit on its lower end by a flexible retaining member (snap ring) that was removable from the assembly. This design had more of a positive retention capability of the Meyers design (above) but is difficult to assemble and disassemble in field applications.
WO Patent # 98/05476 (Moir & McInnes; SDS Digger; 1998) utilizes several split sleeve retainer designs that are also attached between the driver-sub and hammer housing on its upper end and below the bit circumferential shoulder on its lower end. These split retainers may be supported by an additional outer ring and/or sleeve to prevent the split sleeves from moving radially away from the circumferential retaining shoulder. In addition, this patent also shows a combination retainer sleeve and driver sub with extended fingers design which is assembled through milled slots in the retaining shoulder. The milled slots are used to allow assembly and to also prevent rotation of the bit to a position of which the retainer lugs will drop through the milled slots in the retaining shoulder.
U.S. Pat. No. 6,070,678 (Pascale; Numa Tool Co.; Jun. 6, 2000) utilizes a retainer sleeve which is attached between the driver-sub and hammer housing on its upper end and slides past the bit retaining shoulder. After the lower section of the retainer is in position below the bit retaining shoulder, pins are driven into the retainer which effectively prevent the retainer sleeve from moving up past the bit's retaining shoulder. These pins are “fixed” to the retainer until disassembly.
An additional patent for a retention system includes U.S. Pat. No. 6,021,856 (Pascale, Jack H.). While each of the above is unique in design, none of these provide the features of the current invention.
Thus, there is a need for a device for retaining and retrieving broken percussion drill bits. Further, it is advantageous for such a device to make connecting to the broken piece easy by retaining a connection, have a means for resisting disconnecting, and protecting the assembly as it is removed from the hole.